A backflow preventer is used to prevent possibly contaminated water from a service line from flowing back into a main line in case a reverse pressure differential occurs between the service line and main line. Thus if the pressure in the main line drops precipitously, due for example to use of a nearby fire hydrant, so that its pressure falls below that of a service line water will not be sucked by the main line back from the service line. Similarly if the pressure downstream of a backflow preventer increases, as for instance when a boiler having an automatic-fill connection to the water system fails and overheats, the backflow preventer should prevent this certainly unsavory water from reentering the drinking-water main.
Typically such a valve has a downstream or outlet chamber connected to the service line and provided with a simple spring-loaded check valve that substantially blocks reverse flow back from the service line. Its downstream or inlet side is connected to the normally high-pressure main and its center has an externally open vent that is downstream of the outlet-chamber check valve. Some sort of valve structure is provided in a downstream or inlet chamber formed in the valve housing and into which the main and the vent open and which is partially defined by the downstream end of the check valve. The valve body in this inlet chamber is actuated by pressure to vent the downstream chamber when pressure in the main line drops below a predetermined level, thereby creating in theory a zero-pressure compartment between the main and service line, both of which are under some pressure, so that fluid exchange between them is virtually impossible.
In order also thus to sever the connection when, for instance, flow in the main line is suddenly stopped without a corresponding rapid decrease in pressure, it is preferred to provide some sort of bleed arrangement which reduces pressure at least upstream of the valve as a safety precaution. German patent document No. 2,418,035 (equivalent U.S. Pat. No. 3,903,916) describes a bleed passage, and German patent document No. 2,747,941 a bleed valve used for this purpose.
Typically such valves are complicated and expensive. A separate hydraulic mechanism or electric energization is needed for operation. Furthermore they are known to jam and fail by not shutting down the line when necessary, due mainly to a failure of delicate mechanism. A final problem with such backflow-preventing valves is that they constitute a considerable obstruction to flow themselves, so that they must be dimensioned quite large even to control a small line in order to permit low-resistance flow.